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HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - February 2007

1. Northern Hemisphere

       The 500-hPa height pattern during February featured positive anomalies over the high latitudes of the central North Pacific Ocean , Greenland , the polar region, the subtropical North Atlantic Ocean , and eastern Asia , and negative anomalies from eastern North America to Great Britain , and across central and eastern Siberia (Figs. E9, E11). The main surface temperature departures reflected warmer-than-average conditions across central and southern Europe , and the southern half of Asia , and cooler-than-average conditions across most of North America and the northern half of Eurasia (Fig. E1). The main precipitation anomalies included above-average totals across most of central and southern Europe , and below-average totals in the southeastern U.S. , southeastern Alaska , and northwestern Russia (Fig. E3).

a. Pacific/ North America

The middle- and upper-level circulation during February featured an amplified trough over eastern North America , and a mean ridge over western Alaska (Fig. T22). This pattern was associated with anomalously cold surface temperatures across Canada and the eastern half of the United States , with departures in many areas averaging -2C to -3C (Fig. E1). The most significant departures occurred in the upper-Midwest and mid-Atlantic regions of the U.S. , where mean temperatures were generally in the lowest 10th percentile of occurrences.

In Alaska the monthly temperature and precipitation patterns strongly reflected the position of the mean upper-level ridge. Anomalously warm temperatures in western Alaska coincided with the ridge axis, and anomalously cool conditions in eastern Alaska coincided with the area downstream of the ridge axis. Anomalous descending motion and exceptionally dry conditions were observed across southern Alaska within and immediately downstream of the mean ridge axis (Fig. E3).

b. North Atlantic / Europe

The 500-hPa circulation during February featured a 3-celled anomaly pattern with by positive height anomalies over Greenland and across the subtropical North Atlantic , and negative height anomalies extending from the eastern U.S. to Great Britain (Fig. E9). This pattern reflected the combination of the negative phase (-0.5) of the North Atlantic Oscillation (NAO) and the strong positive phase (+ 1.7) of the East Atlantic teleconnection pattern (Table E1, Fig. E7). These patterns contributed to cooler than average conditions across northern Europe , and warmer and wetter than average conditions across central and southern Europe .

c. Asia

A pronounced north-south dipole pattern of 500-hPa height and surface temperature anomalies was evident across Asia . Below-average heights and anomalously cool surface temperatures extended across northern Russia and Siberia, with temperatures in many regions averaging 2C-3C below normal and in the lowest 30th percentile of occurrences. In contrast, surface temperatures were generally 3-6C above average across east-central Asia , in association with a persistent 500-hPa ridge centered just north of the region. For central and eastern China , this is the second straight month with temperatures in the upper 10th percentile of occurrences.

2. Southern Hemisphere

The 500-hPa circulation pattern during February featured positive height anomalies in the area south of Australia and over the high latitudes of the eastern South Pacific Ocean, and negative height anomalies just downstream of southern South America (Fig. E15). In Australia a persistent upper-level ridge in the west and trough in the east (Fig. T22) contributed to extremely warm temperatures across the western half of the continent, with many areas recording values in the upper 10th percentile of occurrences (Fig. E1). This anomalous circulation also contributed to well below-average precipitation throughout the continent, with totals in the central and northwestern regions falling in the lowest 10th percentile of occurrences (Fig. E3).

The southern Africa rainy season extends from October to April. Precipitation during February was below average, with the most significant departures observed in south-central South Africa where totals were in the lowest 10th percentile of occurrences. So far during the 2006-07 rainy season, area-averaged totals were below-normal in October and February, near-normal in November and December, and above normal in January (Fig. E4).



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